1. Field of the Invention
This invention relates to multi-charge munitions incorporating hole boring charge assemblies, in particular hole-boring charge assemblies capable of penetrating concrete targets.
2. Discussion of Prior Art
It is known that the attack, disruption, and destruction of fixed targets such as airfield runways, shelters, bunkers, bridges, roadways, railway marshalling yards and dockyards may be effected by first emplacing and then detonating relatively small quantities of high explosive within or under the target. The materials of construction of these targets are typically strong in compression and yet weak in tension, as exemplified by most forms of concrete. Such emplacement exploits both the inherent confining effect of the target material on the charge of emplaced explosive and the tensile weakness of the target material, and futhermore enhances the transmission of energy from the detonated explosive into the immediately adjacent confining medium and onwards into the outlying and underlying target structure.
One known technique of rapid implantation and detonation of explosive charges into fixed targets is to first breach the surface of the target with a hole-boring charge of explosive before driving a secondary charge of explosive into or through the hole so formed, and thereafter initiating detonation of the secondary charge. This technique has the advantage that it may be used in both the manual demolition of fixed targets, in which the hole boring and secondary charges will usually be brought separately and sequentially to the target, and in the attack of such structures by remotely-delivered munition systems such as aerially-deliverable bombs, missiles and shells which systems incorporate both types of charge and a suitable delay device for initiating detonation of the secondary charge.
The main requirement for a hole boring charge as applied to fixed targets is that it should be capable of producing a breach in the target of sufficient width and depth of penetration to permit subsequent emplacement of the secondary charge at a position which will cause enhanced damage to the structure once the secondary charge is detonated. The hole may be large enough to permit complete emplacement of the secondary charge within or even under the target. Alternatively, it may only be large enough to permit a remotely delivered secondary charge to lodge partly in the hole, but this at least has the advantage that it prevents ricochet of the secondary charge away from the target before detonation. In a remotely-delivered munition system in particular, the hole-boring charge should also preferably be of relatively small size and weight in comparison with that of the secondary charge because it is for the most part the latter charge which performs the task of destroying the target.
These requirements have in the past been met in part by the use of a hollow explosive charge having a conical concavity in one face lined with a non-explosive liner. The hollow and secondary charges are configured in what is known as a follow-through munition, in which the hollow charge is positioned axially in front of the secondary charge. When the hollow charge is detonated, the liner collapses upon its axis and is formed into a high velocity jet which upon impact with the target produces a hole. The secondary, follow-through charge is thrust into the hole so formed, either by virtue of its own forward momentum if sufficient to overcome blast-back forces from the hole, or under the influence of an auxiliary charge positioned to the rear of the follow-through charge. However, such known hollow charges fail to fulfil all the requirements for a variety of reasons.
Hollow charges with concavities having acutely-angled apexes generally collapse the liner into long, narrow, high speed jets. These are capable of penetrating both massive structures and armour to considerable depths. However the resulting holes bored in the target material tend to be narrow and tapered and so are not suitable for the subsequent emplacement of a blasting charge therein. The diameter of the hole can be increased by increasing the diameter of the hollow charge, but the corresponding increase in weight of the hollow charge is undesirable and furthermore the increase in target penetration in targets of finite thickness such as concrete walls, roads and runways may cause the secondary, follow-through charge to be emplaced beyond the depth at which it can cause maximum damage to that target.
Wider holes are also produced for the same calibre of hollow charge using shallower angled, lined concavities (ie concavities with large-angled apexes, of apex angles generally greater than 80°, especially greater than 100°) which generally form the liners into projectiles which tend towards lower velocity, non-jet penetrators. However, the shorter lengths and lower kinetic energies of these penetrators result in a significant reduction in performance especially against concrete targets, necessitating an undesirably large charge mass in order to excavate a hole of sufficient volume to permit emplacement of the secondary, follow-through charge to an optimum depth.
The need for a relatively large mass of explosive in the hole-boring hollow charge reduces the weight of explosive which can be used in the secondary, follow-through charge for a given overall weight of multi-charge munition, and when the hole-boring charge is detonated consequently gives rise to excessively large forces on the follow-through charge which may damage the follow-through charge and/or its fuzing system.